Optical head

ABSTRACT

An optical head for recording and reproducing information on and from an opto-magnetic record medium by projecting a light beam onto the record medium and introducing a return light beam reflected by the record medium onto photodetectors, including a light source for emitting a linearly polarized light beam, an objective lens for projection the light beam onto the record medium, and an optical element having a surface on which a beam splitting surface is provided in the diverging or converging return beam. Said beam splitting surface is curved such that light rays of the return light beam are made incident upon the beam splitting surface at substantially the same incident angle, so that the return light beam can be split accurately into P-polarized and S-polarized light beams.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical head for recording andreproducing information on and from an optical record medium such as anopto-magnetic record medium.

2. Description of the Related Art

FIGS. 1 and 2 show a general construction of a known optical head foruse in an apparatus for recording and reproducing information on andfrom an opto-magnetic record medium, in which a linearly polarized lightbeam emitted from a semiconductor laser 50 is made incident on a beamsplitter 51 as a P-polarization light beam, and a part of this lightbeam is transmitted through the beam splitter 51 and is converted into aparallel light beam by a collimator lens 52. The parallel light beamemanating from the collimator lens 52 is made incident upon a reflectionmirror 53 which is inclined by 45 degrees with respect to a plane of thedrawing of FIG. 1, so that the light beam is deflected by 90 degrees andis made incident upon an objective lens 54. Then, the parallel lightbeam iS focused by the objective lens 54 onto a recording surface of anopto-magnetic record medium (not shown) as a fine spot.

In the opto-magnetic record medium information is recorded as amagnetizing direction, and when the linearly polarized light beam isreflected by the record medium, the polarizing direction is rotated inopposite directions depending on the magnetizing direction by the wellknown Kerr effect. The reflected light (return light) reflected by theopto-magnetic record medium again travels back along the optical axisonto the beam splitter 51 through the object lens 54, reflection mirror53 and collimator lens 52.

The return light beam impinging upon the beam splitter 51 includes anS-polarization component, because the polarization plane of this lightbeam-has been rotated by the Kerr effect. A substantial part of theS-polarization components is reflected by the beam splitter 51, and asmall part of a P-polarization component is reflected by the beamsplitter 51.

The return light reflected by the beam splitter 51 is transmittedthrough a concave lens 55 and is then transmitted through a halfwavelength plate 56, so that the polarizing direction of this light beamis rotated by 45 degrees. Then, the light beam emanating from the halfwavelength plate 56 is made incident upon a polarization beam splitter57. The thus introduced beam is split into the P-polarization componentand the S-polarization component by a flat beam splitting surface 57a inthe polarization beam splitter 57. The P-polarization componenttransmitted through the beam splitting surface 57a is made incident upona first light receiving element 58a of a photodetector 58, and theS-polarized component reflected by the beam splitting surface 57a andfurther reflected by a reflection surface 57b of the polarizing beamsplitter 57 is made incident upon a second light receiving element 58bof the photodetector 58. By suitably processing output signals generatedfrom the first and second light receiving elements 58a and 58b, it ispossible to derive an information signal, a focusing error signal and atracking error signal.

FIG. 2 is a plan view of the photodetector 58 having the light receivingelements 58a and 58b, each of which is divided into three strip-shapedlight receiving regions. The principle of detecting the above mentionedsignals is well known in the art, so that its explanation is omittedhere.

In the conventional optical head, the polarization beam splitter 57 forsplitting the return light reflected by the opto-magnetic record mediumis arranged in a converging light beam. Therefore, incident angles oflight rays impinging upon the flat beam splitting surface 57a are notconstant, but are different from each other. Therefore, it is impossibleto accurately split the incident light beam into the P- and S-polarizedcomponents. This causes a drawback in that the above mentioned signalsobtained from an output of the photodetector 58 might be deteriorated.

SUMMARY OF THE INVENTION

The present invention has for its object to provide a novel and usefuloptical head, in which the return light beam reflected from theopto-magnetic record medium can be split accurately into P- andS-polarized components by means of a polarizing beam splitter which isarranged in the converging light beam, and thus the deterioration of thesignals can be reduced.

According to the invention, an optical head for recording andreproducing information by irradiating a light spot on an optical recordmedium comprises:

a light source for emitting a linearly polarized light beam;

an optical lens system including an objective lens for projecting saidlight beam onto the optical record medium as a fine spot and forcollecting light reflected by the optical record medium to form a returnlight beam;

a beam splitting means having a beam splitting surface arranged in a(converging or diverging) light beam for splitting said return lightbeam into a plurality of light beams, said beam splitting surface beingcurved such that light rays in the return light beam are made incidentupon said beam splitting surface substantially at the same incidentangle; and

a photoelectric converting means having a plurality of photodetectorsfor receiving said plurality of light beams split by said beam splittingsurface.

According to the invention, said beam splitting means comprises thecurved beam splitting surface, so that although the beam splittingsurface is arranged in the diverging or converging light beam, the lightbeam can be made incident upon the curved beam splitting surfacesubstantially at the same incident angle. Therefore, the incident beamcan be split accurately, and thus not only the information signal, butalso the focusing and tracking error signals can be obtained precisely.

In Japanese Patent Application Laid-open Publication Kokai Hei 3-23413,there is disclosed a known optical head, in which a diffraction gratingserving as the beam splitting means is arranged on a curved surface ofan spherical objective lens. Further, in Japanese Patent ApplicationLaid-open Publication Kokai Hei 4-109435, there is also described aknown optical head, in which a beam splitting surface formed by a halfmirror is curved in order to introduce the astigmatism into the lightbeam. However, in these known optical heads, it is not mentioned at allthat the beam splitting surface is covered such that almost all lightrays in an incident light beam are made incident upon the beam splittingsurface at substantially a same incident angle, although the beamsplitting surfaces are arranged within the converging light beams.

Brief Description of the Drawings

FIG. 1 is a schematic plan view showing the construction of the knownoptical head;

FIG. 2 is a plan view illustrating the photodetector shown in FIG. 1;

FIG. 3 is a sectional view depicting a first embodiment of the opticalhead according to the invention;

FIG. 4 is a plan view showing the silicon substrate of the firstembodiment;

FIG. 5 is a graph representing the reflection characteristic of thepolarizing beam splitter;

FIG. 6 is a sectional view illustrating the construction of a secondembodiment of the optical head according to the invention;

FIG. 7 is a plan view of the first photodetector of the secondembodiment;

FIG. 8 is a plan view of the second photodetector of the secondembodiment;

FIG. 9 is a sectional view depicting the construction of a thirdembodiment of the optical head according to the invention;

FIG. 10 is a plan view of the silicon substrate of the third embodiment;and

FIG. 11 is a schematic view showing the hologram of the thirdembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a schematic sectional view showing a first embodiment of theoptical head according to the invention and FIG. 4 is a plan view of asilicon substrate provided in the optical head shown in FIG. 3. On asilicon substrate 1 there are mounted a semiconductor laser 2 and areflection mirror 3 such that a laser light beam emitted by thesemiconductor laser 2 is made incident upon the mirror 3. The light beamemitted from the semiconductor laser 2 is linearly polarized. It isassumed that the polarizing direction, i.e. a direction of the plane ofpolarization is assumed to be an X direction which is perpendicular to aplane of a drawing of FIG. 3. Further, it is assumed that information isrecorded on the opto-magnetic record medium in the form of a trackextending in parallel with or perpendicular to the X direction.

An optical element 6 is disposed for receiving the light beam reflectedby the reflection mirror 3. A hologram 7 is formed in a surface of theoptical element 6 opposite to the semiconductor laser 2. The hologram 7includes a function of diffraction gratings parallelly extending in adirection which is inclined with respect to the X direction by 45degrees (in FIG. 4, an angle measured from the X direction in theanti-clockwise direction is assumed to be a positive angle).Furthermore, the hologram 7 exhibits a lens performance for providingpositive and negative powers to +1 and -1 order diffracted light beams,respectively as will be explained later.

In a surface of the optical element 6 facing the semiconductor laser 2,there is formed a curved beam splitting surface 8. A multi-layered filmmade of dielectric materials is coated on the beam splitting surface 8such that a transmissivity of the beam splitting surface 8 isapproximately 100% for light rays which are made incident upon thesurface substantially at incident angle of zero degree. Reflectingcoefficients of the beam splitting surface 8 for the P-polarized lightand S-polarized light are approximately 0% and 100%, respectively whenthese light beams are made incident upon said surface at an incidentangle of approximately 39 degrees. This designates that the beamsplitting surface 8 functions as a polarizing beam splitter for thelight beam having an incident angle of approximately 39 degrees.

FIG. 5 is a graph representing the reflection property of the beamsplitting surface 8 for the P-polarized and S-polarized light beams,while the incident angle with respect to the beam splitting surface 8 isvaried from zero degree to 45 degrees.

As shown in FIG. 4, photodetectors 4 and 5 are disposed on the siliconsubstrate 1 such that they are aligned on a line which is inclined withrespect to the X direction by -45 degrees and are arranged symmetricallywith respect to the reflection mirror 3. The photodetector 4 is dividedin a direction parallel with said line into three strip-shaped lightreceiving regions 4a, 4b, and 4c. The photodetector 5 is similarlydivided into three strip-shaped light receiving regions 5a, 5b, and 5c.These light receiving regions are also extending in the direction whichis inclined by -45 degrees with respect to the X direction.

As illustrated in FIG. 3, on the surface of the optical element 6 wherethe hologram 7 is formed, there are arranged photodetectors 9 and 10 onrespective sides of the hologram 7. Further, an objective lens 11 isarranged in an optical path of the zero order light beam emanating fromthe optical element 6. The objective lens 11 converges this light beamand projects a fine light spot on a record surface of an opto-magneticrecord medium 12.

In operation, the linearly polarized light beam emitted from thesemiconductor laser 2 (this polarizating direction is in parallel withthe X direction) is made incident on the reflection mirror 3 and isreflected thereby in a direction substantially perpendicular to theplane of the silicon substrate 1. The reflected light beam is then madeincident upon the curved beam splitting surface 8 of the optical element6 at an incident angle of approximately zero degree, so that almost allthe incident beam is transmitted through the beam splitting surface 8.The transmitted light beam is further made incident upon the hologram 7.The zero order light beam emanating from the hologram 7 is directed tothe objective lens 11 and is converged thereby. The thus converged lightbeam is made incident upon the opto-magnetic record medium 12 as a verysmall light spot.

As stated above, the light beam emitted from the semiconductor laser 2is linearly polarized in the X direction. When the light beam isreflected by the opto-magnetic record medium 12, the polarizingdirection is rotated in opposite directions due to the Kerr effect inaccordance with the magnetizing direction in the record layer of theopto-magnetic record medium 12. The light beam reflected by theopto-magnetic record medium 12 (return light beam) is again madeincident upon the optical element 6 through the objective lens 11. Thisreturn light beam is diffracted by the hologram 7 into zero order andhigher order diffraction beams. In the present embodiment, among thesediffraction beams, +1 and -1 order diffracted light beams are used.

As stated above, the gratings of the hologram 7 are inclined by 45degrees with respect to the X direction, and thus the +1 and -1 orderdiffraction light beams emanate in directions which are inclined withrespect to the X direction by -45 degrees. These +1 and -1 orderdiffraction light beams emanating from the hologram 7-are made incidentupon the polarization beam splitting surface 8. In the presentinvention, said beam splitting surface 8 is curved such that all lightrays in the +1 and -1 order light beams are made incident upon thecurved surface substantially at the same incident angle equal to 39degrees. Therefore, almost all the P-polarized component of the incidentlight beams is transmitted through the curved polarizing beam splittingsurface 8 and is detected by the photodetectors 4 and 5 and almost allthe S-polarized component is reflected by the surface 8 and is madeincident upon the photodetectors 9 and 10 through the optical element 6.

Due to the refracting function of the hologram 7, the +1 orderdiffracted light beam received by the photodetector 4 is focused at apoint before the photodetector 4, but the -1 order diffracted light beamimpinging upon the the photodetector 5 is focused at a point behind thephotodetector 5. The positive and negative refracting powers of thehologram 7 are set such that when the objective lens 11 is positioned atan in-focus point in which the light beam is just focused on the recordlayer of the opto-magnetic record medium 12, light spots on thephotodetectors 4 and 5 have the same size. Therefore, when the objectivelens 11 is in a de-focus position which is closer to or far from theopto-magnetic record medium 12, the light spots on the photodetectors 4and 5 vary in opposite senses. That is to say, when the size of the spoton the photodetector 4 becomes larger or smaller, the size of the spoton the photodetector 5 becomes smaller and larger, respectively.

Therefore, a focusing error signal FE can be obtained by the known beamsize method by calculating the following equation:

    FE=(Ia-Ib+Ic)-(Ja-Jb+Jc)

where Ia, Ib, Ic and Ja, Jb, Jc represent output signals of lightreceiving regions 4a, 4b, 4c and 5a, 5b, 5c of the respectivephotodetectors 4 and 5 shown in FIG. 4.

A tracking error signal TE can be derived from the following equation bythe also known push-pull method:

    TE=(Ia-Ic)-(Ja-Jc)

A rotation of the polarizing direction of the return light beam withrespect to the X direction due to the Kerr effect causes a variation ina ratio of an amount of the P-polarization component to an amount of theS-polarization component which are separated by the polarizing beamsplitter surface 8, and thus an information signal S can be derived fromthe following equation:

    S=(Ia+Ib+Ic+Ja+Jb+Jc)-(Ka+Kb)

where Ka and Kb represent output signals of the photodetectors 9 and 10respectively.

FIGS. 6 to 8 illustrate a second embodiment of the optical headaccording to the invention, where portions similar to those of theprevious embodiment are denoted by the same reference numerals used inthe first embodiment. A beam splitter 13 is provided on the optical pathof the light beam emitted by the semiconductor laser 2, and a collimatorlens 14 is provided on the optical path of the light beam transmittedthrough the beam splitter 13. A reflection mirror 15 is provided on theoptical path of the light beam emanating from the collimator lens 14 forchanging a propagating direction of the light beam upward in FIG. 6 andis made incident upon a recording surface of the opto-magnetic recordmedium (not shown) as a fine spot by means of an objective lens 16.

A light beam reflected by the opto-magnetic record medium (return lightbeam) is made incident on the light beam splitter 13 through theobjective lens 16, mirror 15 and collimator lens 14 successively. Thelight beam reflected by the light beam splitter 13 is then made incidentupon a half wavelength plate 17. The light beam emanating from the halfwavelength plate 17 is then made incident upon an optical element 6.Light beams separated by the optical element 6 are made incident uponfirst and second photodetectors 18 and 19.

As illustrated in FIG. 7, the first photodetector 18 has four lightreceiving regions 18a to 18d which are divided into two orthogonaldirections which are in parallel with and perpendicular to theinformation track on the opto-magnetic record medium. As shown in FIG.8, the second photodetector 19 has two light receiving regions 19a and19b which are divided by a line extending in parallel with theinformation track on the opto-magnetic record medium.

In the present embodiment, the optical element 6 has the curved beamsplitting surface 8 formed on an incident-side surface of the opticalelement 6. The beam splitting surface 8 is coated with the multi-layeredfilm of dielectric materials (not shown) to have such an opticalproperty that a reflecting coefficient for the P-polarized light beam isapproximately 0% and that for the S-polarized light is approximately100% at a predetermined incident angle of α. That is to say, the beamsplitting surface 8 functions as an ideal polarizating beam splitterwhen the light beam is made incident thereupon at the incident angle α.

In operation, the linearly polarized light beam emitted from thesemiconductor laser 2 is made incident on the beam splitter 13, and thelight beam transmitted through the beam splitter 13 is converted intoparallel beams by the collimator lens 14, and thereafter the propagatingdirection of the parallel light beam is changed by 90 degrees by themirror 15. Then, the light beam is made incident upon the recordingsurface of the opto-magnetic record medium as a very fine spot by meansof the objective lens 16.

On the opto-magnetic record medium, the information is recorded as themagnetizing direction, and when the linearly polarized light beam isreflected by the opto-magnetic record medium, the polarizing directionis rotated in opposite directions by the Kerr effect depending on themagnetizing direction. The light beam reflected by the opto-magneticrecord medium is made incident on the beam splitter 13 through theobjective lens 16, reflection mirror 15 and collimator lens 14. Thepolarizing direction of this return light beam is rotated over 45degrees by the half wavelength plate 17. Then, the return light beam ismade incident upon the curved beam splitting surface 8 of the opticalelement 6. The beam splitting surface 8 is curved such that all lightrays in the return light beam are made incident upon the surfacesubstantially at the predetermined incident angle of α. Therefore, theP-polarized component of the return light beam is transmitted throughthe beam splitting surface 8. In this case, the optical element 6 isarranged such that its optical axis is inclined with respect to theincident light beam, so that the astigmatism is introduced into theP-polarized component transmitted through the optical element 6. TheP-polarized component having the astigmatism introduced therein is thenmade incident upon the first photodetector 18. Almost all theS-polarized component in the return light is reflected by the beamsplitting surface 8 and is then made incident upon the secondphotodetector 19.

Also in the present embodiment, the information signal S can be obtainedfrom the following equation:

    S=(Ia+Ib+Ic+Id)-(Ja+Jb)

where Ia, Ib, Ic, Id represent output signals of the light receivingregions 18a, 18b, 18c, 18d of the first photodetector 18, and Ja, Jbrepresent output signals of the light receiving regions 19a, 19b of thesecond photodetector 19.

Further, the focusing error signal FE can be derived by the followingequation by the astigmatism method:

    FE=(Ia+Ic)-(Ib+Id)

The tracking error signal TE can be obtained from the following equationby the push-pull method:

    TE=(Ja-Jb)

FIGS. 9, 10 and 11 show a third embodiment of the optical head accordingto the invention. In the present embodiment, semiconductor laser 22 andreflection mirror 23 are arranged on a silicon substrate 21. A linearlypolarized light beam emitted from the semiconductor laser 22 is madeincident upon an opto-magnetic record medium 27 via mirror 23, opticalelement 24, collimator lens 25 and objective lens 26, and the returnlight beam reflected by the opto-magnetic record medium 27 is madeincident upon the silicon substrate 21 via the objective lens 26,collimator lens 25 and optical element 24. It should be noted that thelaser light beam emitted by the semiconductor laser 22 is polarized onthe X direction and the information track on the opto-magnetic recordmedium 27 extends in a direction which is inclined with respect to the Xdirection by 45 degrees.

FIG. 10 is a plan view of the silicon substrate 21. On the siliconsubstrate 21 there are provided, in addition to the semiconductor laser2 and reflection mirror 23, eight photodetectors 28a, 28b, 28c, 28d,28e, 28f, 28g and 28h. The photodetectors 28a, 28c, 28e and 28g arealigned on a line which is inclined from the X direction by 45 degrees,and photodetectors 28b, 28d, 28f and 28h are aligned on a line which isinclined from the X direction by -45 degrees. These photodetectors arearranged symmetrically with respect to the reflection mirror 23.Further, the photodetectors 28b and 28d aligned on the line of -45degrees are divided into three light receiving regions 28b-1 to 28b-3and 28d-1 to 28d-3, respectively.

The optical element 24 is formed by two plastic mold lenses 24a and 24bwhich are adhered to each other by means of an optical adhesive agent.The first and second plastic mold lenses 24a and 24b are formed in arotation-symmetrical shape about the optical axis of the optical element24. On a surface of the first plastic mold lens 24a facing thecollimator lens 25, there is formed holograms 29a and 29b. As shown inFIG. 11, the holograms 29a and 29b are divided along a line which passesthrough the optical axis and extends in a direction which is inclined by-45 degrees with respect to the X direction. The hologram 29a has thegrating construction which extends in a direction inclined by 45 degreeswith respect to the X direction, and the grating contraction of thehologram 29b extends in a direction which is inclined with respect tothe X direction by -45 degrees. Therefore, the gratings of theseholograms 24a and 24b are orthogonal relative to each other. It shouldbe noted that the distances of gratings of the holograms 24a and 24b areidentical with each other.

In the present embodiment, a junction surface between these plastic moldlenses 24a and 24b is formed as a polarizing beam splitting surface 30.The beam splitting surface 30 is coated with a multi-layered film ofdielectric materials such that the transmissivity for the light beam atan incident angle of zero degree is approximately 100%, the P-polarizedlight beam having an incident angle of approximately 39 degrees is fullytransmitted through the surface 30 (reflecting coefficient issubstantially 0%), but the S-polarized light beam having the sameincident angle is not transmitted through the surface 30 (reflectingcoefficient is substantially 100%).

On a side surface of the first plastic mold lens 24a facing thecollimator lens 25, an aluminum coating is applied to form a totallyreflecting surface 31 which reflects the S-polarized light beamreflected by the beam splitting surface 30.

Furthermore, in the present embodiment, the silicon substrate 21 isinclined by 14 degrees from a plane which is perpendicular to theoptical axis of the optical element 24 and a prism forming thereflection mirror 23 has the apex angle of 38 degrees, so that the lightbeam reflected by the mirror 23 is coincident with the optical axis ofthe optical element 24. Further, since the silicon substrate 21 isinclined in the manner mentioned above, the +1 and -1 order diffractedlight beams emanating from the hologram 29a are focused at points whichsituate before and behind the surface of the silicon substrate 21 whenthe light beam impinging upon the opto-magnetic record medium 27 is justfocused on the recording layer, so that the focusing error signal can beobtained by the beam size method. However, the above mentionedinclination angle of the silicon substrate and apex angle of the prismare shown as only examples, and other angles may also be used, and insuch cases, various angles described below may be changed depending onthe said angles employed.

In the present embodiment, the information track on the opto-magneticrecord medium 27 extends in a direction which is tilted by 45 degreesfrom the X direction, where the X direction represents the linearlypolarized direction of laser light beam emitted by the semiconductorlaser 2.

Now the operation of the optical head of the present embodiment will beexplained. The laser light beam linearly polarized in the X direction isemitted from the semiconductor laser 22, 22 and is made incident uponthe beam splitting surface 30 of the optical element 24 at an incidentangle substantially equal to zero degree. Therefore, the light beam isalmost transmitted through the beam splitting surface 30 and is madeincident upon the holograms 29a and 29b. The zero order beam emanatingfrom the holograms 29a, 29b is then converted into the parallel lightbeam by means of the collimator lens 25. This parallel light beam isconverged by the objective lens 26 and is projected onto theopto-magnetic record medium 27 as the fine spot.

When the light beam linearly polarized in the X direction is reflectedby the opto-magnetic record medium 27, the polarizing direction isrotated from the X direction by the Kerr rotation angle in a directiondepending upon the direction of the magnetization in the record layer ofthe opto-magnetic record medium 27. The light beam reflected by theopto-magnetic record medium 27 is then made incident upon the holograms29a, 29b by means of the objective lens 26 and collimator lens 25.

The +1 order beam emanating from the hologram 29a is diffracted in adirection inclined by 135 degrees with respect to the X direction, the-1 order beam emanating from the hologram 29a is diffracted into adirection inclined by -45 degrees with respect to the X direction, andthese first order diffracted light beams are made incident upon the beamsplitting surface 30 substantially at the same incident angle. Almostall the P-polarized component of the +1 order diffraction beam istransmitted through the beam splitting surface 30 and is made incidentupon the photodetector 28b, and almost all the S-polarized component ofthe +1 order diffraction beam is reflected by the beam splitting surface30 and is further reflected by the total reflection mirror surface 31and is finally made incident upon the photodetector 28f. Similarly,almost all the P-polarized component of the -1 order diffraction beam istransmitted through the beam splitting surface 30 and is made incidentupon the photodetector 28d, and almost all the S-polarized component ofthe -1 order diffraction beam is reflected by the beam splitting surface30 and total reflection mirror surface 31 and is then made incident uponthe photodetector 28h.

The +1 order beam of the return beam impinging upon the hologram 29b isdiffracted into a direction inclined by 45 degrees from the X direction,and the -1 order beam is diffracted into a direction inclined by -135degrees with respect to the X direction. These +1 and -1 orderdiffraction beams emanating from the hologram 29b are then made incidentupon the beam splitting surface 30. Almost all the P-polarized componentof the +1 order diffraction beam is transmitted through the beamsplitting surface 30 and is made incident upon the photodetector 28a,and almost all the S-polarized component of the +1 order diffractionbeam is reflected by the beam splitting surface 30 and is furtherreflected by the total reflection mirror surface 31 and is finally madeincident upon the photodetector 28e. Similarly, almost all theP-polarized component of the -1 order diffraction beam is transmittedthrough the beam splitting surface 30 and is made incident upon thephotodetector 28c, and almost all the S-polarized component of the -1order diffraction beam is reflected by the beam splitting surface 30 andtotal reflection mirror surface 31 and is then made incident upon thephotodetector 28g.

As explained above, the silicon substrate 21 is inclined with respect tothe plane perpendicular to the optical axis of the optical element 24 by14 degrees, so that in the in-focused condition, the +1 diffraction beamemanating from the hologram 29a is focused at a point before thephotodetector 28b and the -1 order diffraction beam emanating from thehologram 29a is focused at a point behind the photodetector 28d suchthat the light beam spots formed on these photodetectors 28b and 28dhave the same size. The sizes of these spots on the photodetectors 28band 28d are varied in opposite directions in accordance with thedefocusing.

Therefore, in the present embodiment, the focusing error signal FE canbe obtained by the beam size method in accordance with the followingequation:

    FE=(Ib1-Ib2+Ib3)-(Id1-Id2+Id3)

Where I b1, Ib2, Ib3 and Id1, Id2, Id3 represent output signalsgenerated by the light receiving elements 28b-1, 28b-2, 28b-3 and 28d-1,28d-2, 28d-3 of the photodetectors 28b and 28d, respectively.

The holograms 29a and 29b are divided by the line parallel with theinformation track of the opto-magnetic record medium 27, and thus thetracking error signal can be obtained by the push-pull method byderiving a difference between an amount of light transmitted through thehologram 28a and an amount of light transmitted through the hologram29b. That is to say, the tracking error signal TE can be obtained by thefollowing equation:

    TE=(Ib1+Ib2+Ib3+Id1+Id2+Id3+If+Ih) -(Ia+Ic+Ie+Ig)

where Ia, Ic, Ie, If, Ig and Ih denote the output signals of thephotodetectors 28a, 28c, 28e, 28f, 28g and 28h.

The polarizing direction of the return beam reflected by theopto-magnetic record medium 27 is rotated by the Kerr rotation anglefrom the X direction, and the variation of the polarizing directionresults in the variation of a ratio of the P-polarization component tothe S-polarization component emanating from from the beam splittingsurface 30. Therefore, the information signal RF can be obtained by thefollowing equation,

    RF=(Ib1+Ib2+Ib3+Id1+Id2+Id3+Ie+Ig) -(Ia+Ic+If+Ih)

Advantages of the third embodiment may be listed as follows incomparison with the first embodiment.

(1) The use of collimator lens 25 can provide a separated optical systemin which the objective lens 26 is movable and the remaining members arefixed.

(2) Since the holograms 29a and 29b are formed as the equidistant linearlattice, the +1 and -1 order beams are diffracted in the oppositedirections. Consequently, upon determining the surface configuration ofthe beam splitting surface 30, it is not necessary to consider adifference in the diffraction angle between the +1 and -1 orderdiffraction beams. This results in that both the diffraction beams canbe made incident upon the beam splitting surface 30 precisely at theincident angle of 39 degrees. Therefore, the quality of the informationsignal can be improved.

(3) The optical element 24 is formed by adhering the first plastic moldlens 24a and the second plastic mold lens 24b, and hence the P-polarizedcomponents of the light beam transmitted through the beam splittingsurface 30 is not refracted by the beam splitting surface. This resultsin that only a small aberration is introduced into the P-polarizedcomponent of the light beam transmitted through the optical element 24and thus the quality of the focusing error signal.

(4) The S-polarized component reflected by the beam splitting surface 30is again reflected by the total reflecting mirror surface 31, and thusall the photodetectors can be formed on the single silicon substrate 21.

(5) Since the mirror surface 31 has a curved shape, the diverging lightbeam reflected by the beam splitting surface 28 is converged by themirror surface. Therefore, the photodetectors 28e, 28f, 28g and 28hreceiving the thus converged light beams can be smaller.

The present invention is not limited to the embodiments explained above,but many alternations and modifications may be conceived by thoseskilled in the art within the scope of the invention. For instance, inthe above embodiments, the beam splitting surface is arranged within theconverging light beam, but according to the invention, it is alsopossible to provide the beam splitting surface within the diverginglight beam. Further the half wavelength plate provided in the secondembodiment shown in FIG. 6 may be arranged in the other embodiments.Moreover, in the third embodiment illustrated in FIGS. 9 to 11, thehologram is formed by the two hologram halves which are divided in thedirection of the information track on the record medium, the diffractiongratings of the first and second hologram halves are extended in themutually orthogonal first and second directions, and the first directionis in parallel with said division direction. According to the invention,it is not always necessary to set the first and second directions of thediffraction gratings to be perpendicular to each other and to set thefirst direction in parallel with the division direction.

As fully described hereinbefore, according to the invention, the beamsplitting surface is curved such that almost all light rays of thereturn light beam reflected by the opto-magnetic record medium are madeincident upon the beam splitting surface substantially at the sameincident angle although the beam splitting surface is arranged withinthe converging or diverging light beam, and therefore it is possible toeffect the beam splitting function accurately and to obtain theinformation signal, focusing error signal and tracking error signal in aprecise manner.

What is claimed is:
 1. An optical head for recording and reproducinginformation by irradiating a fine light spot on an optical recordmedium, comprising:a light source for emitting a linearly polarizedlight beam; an optical lens system including an objective lens forprojecting said light beam onto the optical record medium as the lightspot and for collecting light reflected by the optical record medium toform a return light beam; a beam splitting means having a beam splittingsurface for splitting said return light beam into a plurality of lightbeams, said beam splitting surface being curved such that light rays inthe return light beam are made incident upon said beam splitting surfacesubstantially at a same incident angle; and a photoelectric convertingmeans having a plurality of photodetectors for receiving said pluralityof light beams split by said beam splitting surface, wherein said beamsplitting surface is arranged in a converging portion of said returnlight beam.